That depends on what is considered the bug. I consider comparing
string variables
without quotes a dodgy decision at best. One could consider these
string comparisons
without double quotes to be a bug.

You are wrong. If the variables are allowed to be undefined, the correct
approach would be to use :U. Adding random "" is not what make fragments
should use.
Joerg

Fine. Would some makefile expert please fix replace.mk in the
sanctioned way?